The optimal timing of forensic evidence collection following paediatric sexual assault

Forensic evidence collection following sexual assault has an important medico-legal role. Despite the advent of DNA profiling, research into the optimisation of forensic biological specimen collection is limited. This has led to inconsistent and variable guidelines for forensic evidence collection. The guidelines in this jurisdiction (Victoria, Australia) recommends that specimens be collected up to 7 days following sexual assault in some circumstances. The aims of this study were to determine the optimal times post sexual assault for the collection of forensic biological evidence in paediatric cases (aged 0-17 years).

A retrospective review of paediatric sexual assault cases seen by the Victorian Forensic Paediatric Medical Service (VFPMS) between 1 January 2009, and 1 May 2016, was undertaken. Specimen site and collection times post assault were collated from VFPMS medico-legal reports and compared with the forensic evidence analysis results reported by the Victoria Police, Forensic Services Department. In addition, a survey of recommended forensic specimen collection times post assault in the different Australian jurisdictions was undertaken for comparison.

Within the 6 year 5 month period studied there were 122 cases consisting of 562 different forensic specimens that were collected and analysed. 62 (51%) of cases produced one or more positive forensic result and, of the 562 specimens collected, 153 (27%) were positive for one or more of foreign DNA, spermatozoa, semen or saliva. Foreign DNA was more likely to be found if forensic specimens were collected during the first 24 h after the assault as compared with those collected at 25-48 h, (p < 0.005). Similarly, spermatozoa were identified more frequently on swabs collected at 0-24 h compared to 25-48 h (p < 0.002). Foreign DNA was not identified beyond 48 h post assault and spermatozoa were not identified beyond 36 h. Saliva and semen were not identified beyond 24 h. The youngest victims with positive forensic evidence were 2-3 years old. The survey of current forensic specimen collection practice in Australia shows that the guidelines for timing of forensic evidence collection in child sexual assault cases is highly variable between jurisdictions.

Our results highlight the importance of collecting forensic specimens as a matter of urgency, regardless of age, within the first 48 h post assault. Although there is need for further research, the findings indicate a need for the re-evaluation of current guidelines for specimen collection in paediatric sexual assault cases.